I currently have a 1-wire O2 sensor and have read several places that replacing it with a 3-wire heated will speed up response of the sensor showing a more true reading and having beneficial results, I understand that.
I also know the basics on wiring up a 3-wire sensor in the 1-wire system, 1 sensor wire, 1 wire to ground and 1 to ignition positive.

BUT.

What is the 4th wire for on a 4-wire sensor? searching hasn't yielded any positive results as of yet.

I am working on GM vehicles with 1 wire sensors in the 80's era, 1 is an 1981 that will be upgraded to a later model ECM and TBI injection. The others are 87 and 88 respectively. I have a 4-wire from a 1998 GM vehicle and was just wondering if it could be retrofitted as easily as a 3-wire sensor?

Hey M&M, GM 4 wire o2 uses black,pink,tan,purple. Black is ground, pink is ignition hot(these are the heater circuit). Tan is computer reference low, purple is computer reference high (these are the sensor output voltage circuit).The three wire output voltage is internally grounded in the sensor and outputs to the computer with one wire .1v-.9v. The four wire is not internally grounded, its output voltage is fed to the computer with two wires, same voltage (.1-.9). Although I've never tryed it, you should be able to ground the tan wire(ref. low) and input to a single wire computer with the purple wire(ref. high). Hope this answers your question.olnolan

Hey M&M, GM 4 wire o2 uses black,pink,tan,purple. Black is ground, pink is ignition hot(these are the heater circuit). Tan is computer reference low, purple is computer reference high (these are the sensor output voltage circuit).The three wire output voltage is internally grounded in the sensor and outputs to the computer with one wire .1v-.9v. The four wire is not internally grounded, its output voltage is fed to the computer with two wires, same voltage (.1-.9). Although I've never tryed it, you should be able to ground the tan wire(ref. low) and input to a single wire computer with the purple wire(ref. high). Hope this answers your question.olnolan

I'm thinking you might have the grounding thing backwards on the 3-wire, if it were internally grounded it wouldn't need the third wire, similar to the 1-wire internally grounded and sensor resistance is read by the ECM as needed.

I am curious on it though as I have a few 4-wire sensors laying around, I could always hook them up to 12-volts and use the multi-meter to read the output and same thing with a 1-wire, heat it up and check it to compare.

I have a known bad O2 sensor in my 1987 truck I can pull and experiment with, I can use the laptop and program along with the ALDL connector and see what it does messing with one of the 4-wire units.

Hey M&M, Both the one wire and the three wire sensors are internally grounded on the negative side of the cell, look in a repair manual or such and it'll show you the diagram. The o2 sensor doesn't work on variable resistance like say a water temperature sensor. It has a concentrator cell that produces its own voltage from a chemical change that happens as its heated. Approx 0-1v output. The cell compares the oxygen level in the pipe to a given oxygen within the cell and varys the voltage accordingly. But it has to reach about 600 degrees before it starts doing anything. The addition of a heater in the sensor gets it up to temperature faster for quicker response. The only reason I can think of for going to 4 wire sensors is to isolate the signal from ground noise by bringing two wires back to the computer. When you're reading a signal at the millivolt level it doesn't take but a little bit of ground transient noise to throw the voltage off. As a matter of fact I think the newer four wire sensor is also of a different design than the original sensors for improved accuracy. I'm by no means a computerized auto tech. But my Electrical & Instrumentation background helps me in figuring this junk out. Man, the days of simplicity in autos is gone. So far with my late model daily drivers I've been able to fix them myself with a little reading and testing and not have to bring it to the dealer techs. Good luck with yours.olnolan

I was searching the internet yesterday about this and came up with several things posted about the 3-wire sensor being able to wire the sensor terminal with the 1-wire harness connector and using either of the other 2 as 12v+ and 12v- stating it didn't matter as the heating element would work the same either way, being DC voltage fed to it this makes sense.

One manual here shows a heated O2 sensor wired with the pink wire being a 20 amp fused ignition to power the heating element, the pruple wire being the sensor wire to the ECM/PCM and the black 150 wire to chassis ground, and finally the black 451 wire to chassis ground AND the ECM/PCM system ground as well.

A lot of teh O2 sensors do not depend on the case for ground so there are 2 wires for the sensor part and 2 for the heater. I would be careful on the polarity as it could be built with both potential grounds tied together.

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A lot of teh O2 sensors do not depend on the case for ground so there are 2 wires for the sensor part and 2 for the heater. I would be careful on the polarity as it could be built with both potential grounds tied together.

Yep, you're right about that. I've got several manuals from different later Chevrolet cars and trucks. They changed the color code somewhere along the line or on some models. Sometimes it's blk and br on the heater circuit, pur and tan on the sensor circuit. Sometimes its blk and pink for the heater circuit. When I was researching my LT1 swap I found the book wrong on the color code, the color code wound up the same as my '99 5.3l pickup. I've read about 3 wire replacement sensors with 2 blk and 1 pur, where the blacks were the heater circuit and polarity did not matter on the heater element. Also seen heater circuits with black and white. I've also seen a sensor charts showing the 4 wire sensors with the tan and black case grounded and hard wire grounded. So you have to check our sensor leads with an ohmmeter to case ground to know what you're hooking up. olnolan

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